The present invention relates to applications for which electrical links are required, between different electronic circuits. After, the concept of electronic circuit should be understood in its widest sense, that is to say that an electronic circuit can take the form of an electronic module, for example a chip, a micro-electro-mechanical system, usually referred to by the acronym “MEMS”, a packaged integrated circuit, a module of single or stacked printed circuit boards, a three-dimensional module, etc. These links can electrically interlink physically uniform electronic circuits, for example chips, or even physically dissimilar electronic circuits, when the aim is, for example, to electrically link a chip with an interconnection support with a substrate, a printed circuit board, a package, etc. The signals concerned may be of fast digital or even microwave analogue nature.
More particularly, the present invention relates to applications in which the abovementioned electrical links are intended for the transmission of electrical signals occupying a wide frequency band, and/or which are situated in high frequencies in respect of the dimensions of the links to be produced, and/or which exhibit high power levels. The signals concerned may be of analogue or digital nature. It is, for example, considered that high frequencies in respect to the dimensions of the links to be produced observe the inequality ll>3.109/1000.f, ll representing the link length in meters, and f representing the frequency of the transmitted signal, in Hertz.
When this inequality is not satisfied, the link produced is all the more difficult to compensate when the characteristic impedance of the interfaces is low, when the required matching level is high, and when the frequency band of interest is wide. Implanting a matching network is not always possible, either because the space available is insufficient, or because the elements to be interconnected are fixed and cannot be modified.
In order to limit the interfering influence of the link elements, produced for example in the form of wires or tapes, the electronic circuits which have to be electrically linked are placed as close as possible to one another. The fastenings of the link elements are, for example, produced by thermocompressed or thermosonic bonding techniques. Consequently, the dimensions and the tolerances which are associated therewith and which are associated with the positioning of the elements, are reduced, to the detriment of the production costs and manufacturing output.
This drawback is all the more critical when the assemblies concerned are complex and long chains of dimensions are involved. For example, in a relatively simple case where chips or power modules are mounted on heat dissipaters, through cavities produced in a substrate, a chain of dimensions can be defined as the sum of the distance from the pad on the substrate relative to the edge of the substrate, of the distance from the edge of the substrate to the edge of the chip or of the module, and of the distance from the edge of the chip or of the module to the land on the chip or the module. A fine tolerance associated with such a chain of dimensions is in practice feasible, but at the price of necessarily costly manufacturing and inspection methods, and at the risk of low output.
Another problem that arises in this context is linked to the fact that it is sometimes necessary to link components in assemblies in which the latter have fixing points situated at different heights. In such cases, not only does the height difference between the components or circuits increase the link length, but also it proves difficult to produce a ground return.
There are solutions known from the prior art, that are implemented to limit the influence of interference phenomena or the mismatching of the connections.
A first known technique consists in using connection leads, which can take various forms. These connection leads can, for example, be through-spikes, lyres, or even flat leads mounted on the surface of printed circuits. One drawback with this technique is that it is not effective for the transmission of high frequency signals, and for the dissipation of high power levels.
A second known technique consists in using micro-wiring comprising a plurality of conductive wires in parallel, usually two wires. Such a technique is, however, often limited by the surface area available from the lands, the surface area of which is limited by the frequency of the signals to be transmitted. It is also limited by the phenomenon of mutual inductance between the conductive wires.
A third known technique consists in using micro-wiring comprising micro-ribbons. This technique, however, also presents the drawback of being limited by the surface area available on the lands, the surface area of which is limited by the frequency of the signals to be transmitted. Another drawback with this technique is that it is significantly more costly to implement industrially, by comparison with the abovementioned second wired technique.
A fourth known technique consists in using conductive microballs, soldered between metallized lands of modules mounted flipped relative to one another. This technique is known by the technical name “flip-chip”. For example, an electronic chip or a module equipped with a matrix of conductive balls—often referred to by the acronym BGA, which stands for “Ball Grid Array”—mounted flipped on a substrate. This technique is advantageous for very high frequency links, and/or links with a very wide frequency band. However, this technique is costly to implement industrially, and requires additional steps in the process of manufacturing the devices that implement them. Furthermore, this technique presents the drawback of not being effective in terms of heat dissipation, when it is applied to monolithic electronic circuits, of chip type. It may prove effective when it is applied to modules incorporating a heat dissipater, but in such cases the technique proves overall very costly to implement industrially. This technique also presents the drawback of requiring chips or modules that are designed specifically for this type of assembly. Lastly, it presents a drawback associated with the difficulty, even impossibility, to carry out visual inspections on the links after assembly.
A fifth known technique consists in using micro-lands, assembled directly by soldering or by bonding on electronic circuits mounted flipped relative to one another. This technique is similar to the fourth known technique using microballs, described above. For example, an electronic chip or a module equipped with a matrix of metallized micro-lands—often referred to by the acronym LGA, standing for “Land Grid Array”—mounted flipped on a substrate. This technique also makes it possible to produce very high frequency and/or very wide band links. On the other hand, this technique is not effective for ensuring the matching of the differences in expansion coefficients between the different electronic circuits. In a way similar to the fourth technique described above, this technique presents the drawback of not being effective in terms of heat dissipation, when it is applied to monolithic electronic circuits, of chip type. It may also prove effective when it is applied to modules incorporating a heat dissipater, but at the price of very costly implementation. This technique also presents the drawback of requiring chips or modules that are designed specifically for this type of assembly. It also presents a drawback associated with the difficulty, even impossibility, of carrying out visual inspections of the links after assembly, even when some links are produced with lands which rise up on the sides, for example for modules provided with castellations, according to LGA-specific techniques.
A sixth known technique consists in using micro-bump contacts intended for the production of links by thermocompression or by bonding. This technique makes it possible to produce very high frequency links and/or links with a very wide frequency band. However, this technique does not make it possible to ensure an effective heat dissipation. It also presents a drawback associated with the difficulty, even the impossibility, of carrying out visual inspections on the links after assembly.
A seventh known technique consists of tape-automated bonding, usually referred to by the acronym “TAB”. This technique is based on an electronic circuit produced on a thin and flexible substrate, the tracks of which extend beyond and are directly micro-wired to the interconnection bump contacts of the elements to be linked, for example by thermocompression or by collective soldering. This technique allows for a collective link mode, that is to say that all the connection operations for one and the same printed circuit can be carried out simultaneously. The TAB technique makes it possible, for example, to produce links with coplanar transmission lines, of ground/signal/ground type. Such lines present the drawback of being sensitive to dissymetries, of requiring a minimum of six contact points per link, of requiring ground planes of large surface area, as well as great delicacy in the production of the central line, in terms of track width and of separation from the ground lines, in order to obtain typical characteristic impedances of the order of 50Ω.